Method and apparatus for detecting hydrocarbons



Nov. 14, 1944. v R DOAN 2,362,805

METHob AND APPARATUS FOR DETECTING HYDROCARBONS Filed June 17, 1940 aw'dWOD STORAGE TANK 1 h g 3 q gr! 4% 2 INVENTORI R.L.DOANY ATTEi z toseveral per cent.

Patented Nov; 14, 1944 METHOD AND APPARATUS FOR DETECTING HYDROCARBON SRichard L. Doan, Bartlesville, kla., assignor to Phillips PetroleumCompany, a corporation of Delaware Application June 17, 194-0, SerialNo. 341,031

Claims. (Cl. 23-232) This invention relates to the detection andlocation of gas and oil bearing. strata intersected by a bore holeduring the course of drilling operations.

With the introduction of rotary equipment and continuous mud circulationinto drilling practice the problem of invasion of porous formations bydrilling fluid and the consequent mudding oil of potentially productivezones. assumed a much greater importance than it had when cable toolsonly were employed. In many cases the pressure head of mud in the holeis considerably greater than the gas pressure in the reservoir which ittraverses so that very little, if any, of the reservoir fluidescapes'into the hole and hence tothe surface where it could be observedby the driller. Since direct evidence of oil and gas has always been,and, still is, the most dependable criterion of where to test forcommercial production it is highly desirable to retain this avenue ofexploration which, for reasons cited, has been almost closed so'far asvisual evidence is concerned by the methods of rotary drilling. To dothis it is necessary to make use of instruments and techniques capableof revealing much smallerquantities 01' oil and gas than can be detectedvisually and to conduct routine tests on drilling mud during the timethat the well is drilling through the section of'hole it is desired toinvestigate.

It is an object of the present invention to provide a method ofextracting dissolved and ad- 'sorbed gases from drilling mud and testingthem for combustible content. It is a further object to provide a methodof accurately determining the depth from which the gas originated.

- The volume content of combustible gas in drilling mud may range fromnegligible amounts lip If the gas pressure in a petroleum reservoirtraversed by a bore hole is much less than the hydrostatic head ofmud,,no seepage intothe hole can take place and the the method whichwould prevent its use in the upper ranges for such a purpose as, say,the routine testing of mud in high pressure areas to detect dangerousrises in gas concentration which might, if allowed to go uncontrolled,result in a blowout. It is the range of lower concentrations where thegreatest benefits are to be derived from mud analysis because of theimpossibility of get-- ting direct evidence of oil and gas any other wayexcept bycoring, which is costly and limited in application. In thepresent specification the term occluded is used to describe theconditions of retention of the hydrocarbons in the earth sample whethersuch form of retention be absorption. adsorption, or merely structuralenvelopment. Electrical logs at best give only indirect evidence and aresubject to numerous interpretation hazards which, however, should beonly addition of hydrocarbons to the mud will be v due to the actualvolume of gas andoil contained in the cuttings removed by the drill bit.'On the other hand, where the reservoir pressure approximates or ishigher than the mud ressure, seepage can and does occur and in extremecases may be sumcient to bring about a gas-cut condition of the mud oreven blow it out of the hole.

This invention is primarily concerned with those cases in which the gascontent of the mud is too small to be noticeable by visual inspection,although there is nothing in the application of reduced considerably bybeing used in conjunction with the data obtained from mud analysis. Itis realized, of course, that not all gas-bearing strata will becommercially productive but information on the location of such'zonesobtained through routine mudtesting in accordance with this inventionshould result in much better knowledge of where to test.

Mud discharged from the hole in drilling may carry gas in excess of thatwhich it can hold in a dissolved or adsorbed state at atmosphericpressure. In this case the excess will be given off into the air as themud emerges from the hole. It would thus bepossible to obtain someinformation of value by testing the air above the mud stream in thedischarge line for hydrocarbon content. However, this method is subjectto the disadvantage that it is not applicable when the ga content isbelow the point at which the gas' breaks out of the mud readily and itis probable that most of the cases of importance may fall in thiscategory.

To make use of the dissolved and adsorbed gas as an index of formationstraversed by the drill it is first necessary to extract the gas from themud and it is one of the objects of the present invention to describe aprocess for accomplishing this in a rapid and eflicient manner. Afterextraction, a number of diilerent'methods might be used for determiningthe hydrocarbon content the contained gas for purposes of analysis, aconsiderable amount of aeration and loss of gas from the main body ofmud undoubtedly occurs in circulating through the pits at the surface.However, some will remain in the mud and be carried on a return tripdown the hole. The effect of this is to raise the general background ofgas content against which new increases must be observed. The thing thatmakes it possible to obtain useful information on the petroleum contentof subsurface strata by analysis of the discharge mud is that there isapparently little mixing of the mud in coming up the hole so that thegas travels eflectively in slugs whereas in circulating through the mudpits enough mixing occurs to blend the residual gas into the generalbackground. For analytical purposes it may be desirable to know what thebackgroundis from time to time and tests on suction mud just before itenters the mud hog will furnish the information.

The general analytical procedure to be described comprises pumpingmudfrom eitherthe discharge or suction lines on the rig through a supplylinetraversingv the field laboratory, bypassing a definite volume of mudinto the extraction unit, heating this mud in a closed reservoir to anelevated temperature and pressure, flashing the heated content to awater-cooled evacuated chamber, diluting the extracted gas withhydrocarbon-free air to restore the pressure to atmospheric, pushing thediluted gas mixture out of the extraction apparatus through suitabledrying tubes and into the analytical apparatus by the use of water, andfinally measuring the combustible content of the mixture by anelectrical method. A schematic diagram of the equipment layout is shownin the annexed drawing to which the following details are referred. I

Mud from the discharge pipe on the rig enters the intake of pump IIIthrough the line I I and is forced out through the line 12. vA testsample is taken by opening valves l3 and I4 and partially closing valvel5 which causes a portion of the mud stream in l2 to by-pass through thepressure reservoir l6 until it overflows through line 11 to drain l8. Asample of suction mud may be taken in a similar manner through line i 9.

The sample comprising a known volume of mud is isolated in the pressurereservoir l8 by closing all valves leading to the chamber. This chamberis then heated by the gas flame from burner 20, supplied with fuel whichmay be liquefied petroleum gas contained in tank 2|. To improve theheating conditions a jacket of thermal insulation 22 surrounds chamberIS. The pressure inside reservoir I6 is registered on gauge 23 and theheating is continued until it reaches some predetermined valuepreferably between 30 and 55 pounds gauge corresponding to a temperaturerange of approximately 135 and 150 C. Such a temperature is high enoughto be effective in driving off adsorbed gases and low enough to avoidcracking any oil that may be present. Valve 24 in line 25 is thenopened, connecting chamber [3 with a chamber 26 which has previouslybeen pumped to a vacuum by pump 21 acting through valve 28, line 29 andvalve 30. Thedegree of vacuum desired is attained by observing gauge 3|.The rush of steam and mud into the vacuum chamber carries a large amountor heat which is rapidly dissipated by water circulating in a coppercoil 32 surrounding chamber 23 and soldered to it for good thermalcontact. Vacuum chamber 26 should have a volume several times as largeas the heated reservoir l6 and should be located abov it in order thatmud and water can run back after the first rush has subsided and thestream condensed.

The foregoing procedure fulfills the three essential requirements forrapid and thorough extraction of dissolved and adsorbed gases from themud-high temperature, agitation, and vacuum. The next step is to raisethe pressure in the vacuum chamber to atmospheric in order that the gasmixture may be transferred conveniently to the analytical unit, This isdon by admitting hydrocarbon-free air from cylinder 33 through valves 34and 30. The sample is diluted in so doing but this is not importantsince the analytical method possesses ample sensitivity to measure thecombustible content after dilution. Since the volume of gas extractedfrom the mud is negligible as compared to the volume of the vacuumchamber the dilution factor is essentially constant for all samples.

The method of analysis comprises forcing the gas-air mixture out of thevacuum chamber, through drying reagents contained in tube 35 and intothe combustion unit' 36 of an electrical analyzer. This is accomplishedby admitting water through the valve 31. To indicate the water level inthe vacuum chamber'as the gas is bein forced out, the glass gauge 39 isprovided and is connected to the chamber by ordinary packing glands. Thedanger of accidentally overflowing this chamber by failing to turn offthe water at the proper time is eliminated by a float 39 in the top ofthe chamber which closes valve 31 before the water rises high enough todo any damage.

The combustion unit contains two essentially identical platinumfilaments in separate enclosed chambers one of which is sealed in air atatmospheric pressure while the other can be flushed out and filled withthe sample of gas to be tested. The two filaments are heated toincandescence by passing an electric current through them, anddiiferential changes in resistance due to burning the combustible gas onone of them are measured by means of a bridge circuit. This apparatushas been described in applicant's co'pending application, Serial Number298,708 filed October 9, 1939. Calibration of the unit is checked fromtime to time by means of a synthetic combustible gas-air mixture of vknown proportions contained in tank 41.

Facilities provided on the apparatus for washing out the mud afteranalysis and for testing the pressure and vacuum chambers for leaks aras follows: When'the test of a given sample is completed, valve 13 atthe bottom of the lower reservoir is opened and the mud and water fromboth chambers are flushed out through the drain at the bottom of airpressure applied through 29. A water jet 42 in the lower chamberconnecting to the main supply through valve 43 aids in sweeping out anyespecially heavy or viscous muds. High pressure air may be admitted tothis chamber through valve 44 for the purpose of testing for leaks. Thereceptacle 5 at the top of the pressure reservoir and connecting with itthrough valve 46 is provided as an emergency measure so that mud samplesmay be poured into the extraction apparatus if the mud pump should fail.

The percentage of gas in the mud is calculated in terms of the standardgas (either propane or butane) used in calibrating the electricalanalyzer unit. The reading obtained for a test sample on this unit givesthe percentage of combustible gas in the diluted mixture from the vacuumchamber. The actual volume of gas extracted from the mud is equal to theobserved will be a sufliciently good approximation to conpercentagemultiplied by the volume of the vacuum chamber. This in turn divided bythe known volume of mud used in the test gives the volume percentage ofthe gas present in the mud.

The comparison sample is obtained by admitting into tank 41 a measuredquantity of gas from container 2| reduced in pressure at valve 48 toatmospheric pressure and measured in calibration cylinder 49. Storagetank 41 is previously evacuated and the 'hydrocarbonegas in calibrationcylinder 49 is swept into the storage tank by air from compressed airsource 33 thereby forming a synthetic combustible gas-air mixture ofknown proportions in the storage tank.

The method of utilizing the data on gas content in locating gas and oilbearing strata intersected by the drill is as follows, Each time a mudsample is taken for analysis the depth of the drill bit and also therate of drilling are noted. A method of continuous recording wherebyboth of these factors may be conveniently and accurately obtained at anytime is described in co-pending application Serial Number 385,942 filedMarch 29, 1941, by the applicant and Francis W. Crawford. The point oforigin of the gas carried by the mud is assumed to coincide with theposition of the drill bit at the time the test sample passed it. To

determine this point it is necessary to deduct from the observed depthat the time the sample is taken a footage correction which is obtainedby multiplying the drilling rate in feet per hour by the time in hoursrequired for the mud to travel from the bit to the top of the hole.Various methods have been proposed for determining the circulation timeof the mud from the known pumping rate of the mud hog, size of the pipeand v annular space between pipe and walls of the hole, etc. and bydirect experiment through the addition of dyes or certain lightmaterials such as oats or fibrotex to themud and watching for it to comeback. Such methods are fairly satisfactory for determining totalcirculation time but I give no information on how much of this time isspent on the down trip and how much in coming up. It is the latter thatis of importance in calculating the depth correction for mud samples andit is one of the objects of the present invention to provide a method ofdetermining the time required for mud and cuttings to travel from thebottom to the top of the hole.

The foundation of the method is the drilling chart referred to above onwhich is recorded the progress of the bit at all times during thedrilling operations. The slope of the line on this chart represents thedrilling speed, which is determined to a considerable extent by thenature of the formation being drilled. When the bit moves out of a hardlayer such as limestone into one where the drilling is easy and rapid,such, as an unconsolidated sand, the fact will at once become evident onthe drilling chart by a sharp change in slope of the line. Both the timeand the depth at which this change occurred will be accurately marked.If, when such a change of slope occurs,

samples of cuttings are collected at closely spaced intervals andexamined under the microscope, the time at which the first traces of thenewly penetrated layer appear can be noted. The difference between thistime and the time shown on the drilling chart for the break in the curvewill be an accurate measure of the length of time required for the mudto travel up the hole. Ordinarily there will be a number ofopportunitiesto determine the travel time as the hole is being drilled and in betweensuch determinations it sider the circulation time as equal to a constantmultiplied by the depth of the hole, the value of the constant beingredetermined each time an observation is actually made. i

The most convenient method of interpreting the data obtained from mudanalysis is to plot them in the form of a depth log with gas content asabscissa and corrected depth as ordinate. Such a log will show peaks ofhigher gas content marking the position of gas bearing strata, providingthese strata are mudded off sufliciently to stop gas seepage after thebit has passed them, otherwise only the top will be marked and thebackground gas content will be permanently increased.

Even after making the best possible correction for circulation time, thedepth determination in mud analysis has a greater element of uncertaintythan have direct measurements with the drill pipe or steel line. I havefound, however, that this uncertainty as to the depth of the formationresponsible for the gas can be reduced considerably in those cases wherethe gas-bearing zone is also one of easy drilling compared toneighboring strata. The method used is to plot a drilling-rate log tothe same scale as the gascontent log and compare the two. A highcorrelation between certain peaks on both logs will be noted and forsuch peaks the depth of the top and bottom of the gas bearing stratumwill be accurately defined by drill pipe measurement on thedrilling-rate chart. This is an important concept in the presentinvention, since it places the depth determination in mud analysis on amore quantitative basis than is possible by any method which depends ona measurement of mud circulation time.

I claim:

1. A method of treating drilling fluid comprising confining a sample ofthe drilling fluid in a closed vessel, heating the sample in the vesselto a temperature above the boiling point of the same whereby asuperatmospheric pressure in the vessel results, flashing the heateddrilling fluid under pressure into a lower pressure zone, cooling thevaporous eflluent of the flashing step sufliciently to condense watervapor present, adding an oxygen containing gas to the remaining vaporouseflluent to form a mixture, and passing the mixture to a combustion zoneto measure the combustible content.

2. A method of treating drilling fluid comprismg confining a sample ofthe drilling fluid in a closed vessel, heating the sample in the vesselto a temperature above the boiling point of the same whereby asuperatmospheric pressure in the vessel results, flashing the heateddrilling fluid under pressure into a lower pressure zone, coolmg thevaporous eilluent of the flashing step sufficiently to condense watervapor present, further dehydrating the remaining vaporous efliuent, andpassing the remaining vaporous eflluent to a combustion zone to measurethe combustible content. 3. A method of treating drilling fluidcomprising confining a sample of the drilling fluid in a closed pressurevessel, heating the sample in the vessel to a temperature above theboiling point of the same whereby a. superatmospheric pressure in thevessel results, evacuating a second vessel establishing communicationbetween the vessels:

wherebythe vaporous eiiluent of the pressure VapOrolls eflluent, andpassing the remaining vaporous effluent to a combustion zone to measurethe combustible content.

4. Apparatus for testing aqueous drilling i'iuid -for volatilecombustible constituents comprising a pressure vessel constructed towithstand superatmospheric internal pressures, means to heat the vessel,a chamber arranged to collect vapor from the vessel, evacuating means,valved conduit means connecting the evacuating means with the chamber, avalved conduit connecting the upper part of the vessel'with the interiorof the chamber, an analyzer for combustible gases, and means includingvalved conduit means connected to the chamber for supplying waterthereinto and valved conduit means connecting the chamber with theanalyzer whereby combustible gases segregated in the chamber from thevessel may be displaced by the water and passed to the analyzer foranalysis.

5. Apparatus for testing aqueous drilling fluid for volatile combustibleconstituents comprising a pressure vessel constructed to withstandsuperatmospheric internal pressures, means to heat the vessel, a chamberarranged to collect vapor from the vessel, means for cooling the chambersumciently to condense water vapors, evacuating means, valved conduitmeans connecting the evacuating means with the chamber, valved conduitmeans connected to th chamber for supplying air thereinto to increasethe pressure therein, a valved conduit connecting the upper part or thevessel with the interior of the chamber, an analyzer for combustiblegases, and means including valved conduit means. connected to thechamber for supplying water thereinto and valved conduit meansconnecting the chamber with the analyzer whereby combustible gasessegregated in the chamber may be displaced by the water and passed tothe analyzer for analysis.

, RICHARD L. DOAN.

